Well, you already know about the fertilizer part. That's really the only thing it's good for, but it's the best fertilizer you can use. Vampire fish have been in Sims 3 since it was first released, long before any talk of vampires in the game. I haven't caught any of the new Late Night fish, but I doubt they do anything special.
One funny thing I had happen was with the large aquarium from the High End Loft stuff pack. I found that if you put a fish that's the bait for another fish in the same tank, the bait fish will get eaten. I saw the "fight" the two fish had and then the one was gone from the tank. So, don't put Angel Fish in the tank with Deathfish, etc.